Various types of heating units for heating water in spas/hot tubs have been developed. One type of known heater utilizes an electrical resistance element that generates heat when electrical current passes through the electrical resistance element. Heat generated by the electrical resistance element is transferred through electrically non-conductive material to the water from the spa/hot tub as it flows through the heater to thereby heat the water in the spa/hot tub. The chemicals and the like added to the water in a spa or hot tub may create a corrosive environment. The temperature extremes further contribute to creating a relatively harsh operating environment for heaters in such applications.
Also, known electrical heating units may not provide the desired degree of efficiency. Various types of heat pumps have been developed for use in heating swimming pools and the like. Although such heat pumps have been somewhat successful, they are generally too large and bulky for use in a compact spa/hot tub system. Furthermore, heat pumps developed for swimming pools are generally not designed to heat the water to higher temperatures as required for a typical spa/hot tub, and may also not be suitable for use in the uniquely harsh environment of a typical spa/hot tub. Still further, known electrical heaters for spas/hot tubs may have limited power, such that substantial time is required to bring the water in the spa/hot tub up to the desired temperature if the water was cooled after a period of non-use or the like.